A new study shows that smaller babies tend to have lower IQs.While the differences are too small to have any practical significance they might be of importance at a population level.
Researchers at Columbia University have examined over 2500 children at age seven to study the relationship between birth weight and intelligence.They found that on average, IQ at age seven is directly related to birth weight, while factors such as the mother's age, race, education and socio-economic status were also taken into account.
Many studies have shown low birth weight babies (those born weighing less than 2500 grams) have lower IQ test scores at school age - and the smaller the babies at birth, the larger the IQ deficit.
The new study showed the association between birth weight and IQ was stronger in boys than girls, with a 1000g increase in birth weight related to a 4.6 increase in IQ among boys but only 2.8 points in girls.
The authors say differences in IQ of this size are trivial and of no clinical importance to individual children.But since seven times as many children are born in the normal weight range compared to the low range, even small shifts in the distribution of birth weight within the normal range could affect the distribution of intelligence in the population as a whole.
The findings might also help shed light on the connection between foetal growth and brain development.